The present invention, in some embodiments thereof, relates to sleep analysis and, more particularly, but not exclusively, to sleep analysis based on inter beat interval.
In medicine, it takes a long period of observation to further understand the statuses of patients. Sleep medicine has made great progress in the past years, including apnea and some other chronic diseases related to sleep have been paid more attention to. Some medical research has also shown that sleep problems may be one of the causes of hypertension. The growing interest in sleep and its disorders, including their influence on health, well-being and public safety (such as in car accidents) has therefore caused a continuously increasing need to perform sleep investigations for both research and clinical purposes.
It is common to broadly divide the sleep of a normal healthy individual into three states: Wakefulness, Rapid-Eye-Movement (REM) sleep and Non-REM (NREM) sleep. NREM sleep is oftentimes further subdivided into three sleep stages, known as Stage-1, Stage-2 and Stage-3 according to the increasing threshold to the influence of external stimuli. These stages are also known as the depth of sleep. Stage 3 is sometime referred to as slow wave sleep (SWS) and Stages 1 and 2 are combined to define light sleep (LS).
Sleep stages have been traditionally monitored and examined clinically with a polysomnograph (PSG), which provides data regarding the electrical activity of brain, muscles and eye movement during sleep. The PSG data are analyzed according to a gold standard procedure attributed to Rechtschaffen and Kales (R&K) [Rechtschaffen A., Kales A., eds., “A manual of standardized terminology, techniques and scoring system for sleep staging in human subjects”, Washington D.C.: US Government Printing Office, NIH Publication 204, 1968]. The R&K criteria are primarily based on the analysis of three collected bio-signals: (i) electroencephalogram (EEG), (ii) electrooculogram (EOG), and (iii) electromyogram (EMG). The standard procedure is as follows: EEG signals are derived primarily from the cortex of the brain. At the same time an EMG signal which monitors muscle activity, generally from one of the muscles of the mandible (submental) is measured, together with left eye and right eye EOG (signals produced by eyeball movements relative to the skull). These EEG, EMG and EOG signals are conventionally recorded on a multi-channel physiological recorder.
The number of physiologic inputs which are required in the PSG procedure may vary. Typically, the monitored signals include 2-4 EEG leads, 2 EOG leads, 1-3 or more EMG leads (chin, limbs), airflow monitoring, 1-2 respiratory effort leads, oxygen saturation monitoring, 3 electrocardiogram (ECG) leads, body position monitoring and a microphone. Data is stored during the sleep, and the analysis is typically done manually off-line, according to the standard R&K criteria.
U.S. Pat. No. 7,623,912 to Akselrod et al. describes a technique for determining sleep stages from an ECG signal. A series of cardiac R-R intervals is extracted from the ECG signal and decomposed by a time-frequency decomposition. The time-frequency decomposition is used for determine SWS period and sleep-onset period. EMG parameters are also extracted from the ECG signal and are used for determining REM period. Akselrod et al. also discloses a Poincare plot of the R-R intervals and describes technique for determining REM sleep based on the plot. The contents of Akselrod et al. is incorporated by reference as if fully set forth herein.